1. Field of the Invention
The present invention relates to methods for enhancing velocity error detection with Doppler radar. More particularly, this invention pertains to such a method for use in an aircraft that is equipped with an inertial position reference system and a barometric altimeter.
2. Description of the Prior Art
Doppler radar devices operating, for example at 13.3 GHz are employed in both fixed-and rotary-wing aircraft to determine above-ground velocity. Models of the radar energy radiated by an aircraft antenna system include three or four radiation lobes that are accurately determined with respect to the angle of radiation. In addition modulation techniques are employed. In both cases, the antennas are constructed so that any side lobes which may exist are located within the space enclosed by the three or four main radiation lobes. This principle operates well when an adequate proportion of the radio-frequency energy radiated at the (predetermined) angles is backscattered and displaced by the Doppler frequency.
Surface conditions may exist (e.g. water surfaces having little movement, desert or "skiffle-board terrain") which lack adequate backscatter at the required and calibrated radiation angles.
No antenna is free of side lobe radiation. The energy radiated in the side lobes can increase energy conditions. This can result in the radar receiver's locking to the wrong direction of radiation and a correspondingly inaccurate measurement of velocity.
The above-described problem is well known. As a result, the sum of the three (or four) measured and scaled Doppler frequencies has previously been formed with the correct sign to detect such a condition. When operating correctly, the sum is small. It jumps to a large value when one direction of radiation changes or the system is locked to the wrong direction of radiation, (i.e., locked to a side lobe). In the event however, that two corresponding directions of radiation change during the time required for the formation of the sum (which is dependent, in particular, on the modulation method), the above-described check method, known as a "Beam Sum Check" will fail. "Hardware" measures have additionally been employed with regard to antenna design with respect to low energy side lobe radiation to increase the probability of detection of a radar receiver locked to an unwanted side lobe. It is known that side lobe attenuation provides a measure of the probability of an erroneous side lobe lock when the signal/noise ratio is too small for one of the main radiation lobes.
Incomplete detection of lock to unwanted side lobes and the resultant wrongly measured velocities only leads to a deterioration in the accuracy of navigation in pure Doppler navigation systems. Although this may be unsatisfactory, it can generally be corrected and does not lead to catastrophic consequences. If, however, the velocity values supplied by the Doppler radar system are required by the flight control system for executing critical flight maneuvers (e.g. automatic transition to hover flight in a helicopter) the highest possible probability of detection of incorrect velocity values is essential to minimize the danger of a crash.